This invention relates generally to a relief valve and more particularly to a direct-acting, differential piston type relief valve.
Direct-acting, differential piston relief valves are used to provide a quick opening for excessive hydraulic pressure. They are characterized by a structure in which the regulated hydraulic pressure is applied directly to a primary piston area. The primary piston area is annular and measured in terms of the difference in areas resulting from the piston seating area and either the piston outside diameter or a defined smaller internal diameter. By this method, a large seating diameter is achieved, providing good flow capacity, and operating forces are reduced to provide high pressure capability in a reasonably small package. The force exerted on the piston annular area as a result of the hydraulic pressure tends to move the piston away from the source of the pressure, thereby unseating the valve to relieve the pressure. This opening pressure is typically opposed by a spring. Appropriate selection of the spring force and piston areas will provide a relief valve which will theoretically open at a desired pressure and close when the pressure falls below it.
A primary problem with currently available differential relief valves is they are prone to instability at extremely low flows and even at any flow when the downstream or outlet port is pressurized. Low flow instability involves the piston face to seat relationship and the effect of forces exerted on the piston which develop as it moves away from the seat in relieving extremely low flows. Those skilled in the art are familiar with and understand the theorem demonstrated by Daniel Bernoulli relating fluid flow and the principle of conservation of energy. As a relief valve piston leaves the seat, system kinetic energy (V.sup.2 /2 g) changes from zero to some value greater than zero. The theorem dictates that there must be a corresponding decrease in pressure or static energy as kinetic energy increases. This phenomenon is confined to the region between the fixed seat and moveable piston where an orifice is effectively created when the piston begins to move away. Any piston area exposed to this reduced pressure tends to move toward it, causing the valve to close. Once pressure redevelops, the piston moves back again and the cycle is repeated. The result is high frequency oscillation as the valve opens and closes evidenced by an objectionable and potentially destructive scream. What is desired, of course, is a valve which will pass extremely low flows without resulting opening and closing cycles.
Instability which is caused by back pressure at the outlet ports can occur at any flow rate and involves the nature of the forces which are exerted upon the piston and the character of damping achieved by the chamber having fluid displaced by the moving piston. For example, a typical differential relief valve has a piston and seat arrangement at the outlet port with a passage having an orifice passing through the piston to the spring chamber behind the piston. Fluid at the outlet pressure consequently has access to the spring chamber and, if it fills the spring chamber and no air is present, it can provide viscous damping. The magnitude and nature of pressure in the spring chamber can vary widely because its access to downstream pressure is at the piston-seat area of the valve. Flow at this point is at high velocity and turbulent. Piston forces are associated with flow and are unpredictable. All valves require some damping. It may be as little as that afforded by a piston displacing a uniform, viscous fluid. If air is present in the damping chamber, the damping is not reliable. If the chamber is at low pressure, rapid piston movement can aggravate the situation by generating pressures that are less than atmospheric. Air can be pulled out of solution from oil that is present and the system degenerates to the point where instability is unavoidable. Damping with an air-oil mixture in a low pressure environment is questionable at best. To have any effect at all, the orifice size must be extremely small by comparison with that required for pressure viscous damping (where only oil with no undissolved air will pass through the orifice). A valve which has its spring or damping chamber referenced to downstream pressure is, of course, sensitive to flow forces exerted upon the piston ends as turbulent, high velocity fluid exits the valve. While it is possible to achieve conditions of pressure and flow whereby the valve is stable, a change in downstream pressure can change completely the balance of forces which existed prior to the change. For instance, when this type of valve is relieving directly to reservoir, downstream pressure is very low (essentially zero) while spring chamber pressure can be (and frequently is) less than atmospheric. This phenomenom is caused by venturi action (once more, Bernoulli's theorem) as relieved fluid passes the piston end at high velocity. Now, if a restriction is presented to downstream flow, downstream pressure will increase. However, partial vacuum can persist in the spring chamber because venturi action continues, even with rising downstream pressure. When this pressure rises to a level where it overcomes the Bernoulli effect on the piston end, the spring chamber pressure changes suddenly from a partial vacuum to a positive value with a dramatic change in forces which act upon large piston areas. The valve cycles closed and open, violently, at a frequency and magnitude which are proportional to the energy being dissipated.
The industry has endeavored to solve the instability problem in differential piston relief valves by increasing friction and using viscous damping. Some viscous damping is desirable in all valves to assure that a valve has inherent stability. Increasing friction, however, has a detrimental effect on hysteresis.
Hysteresis is represented by graphing pressure and flow rate for a valve as the valve goes from a closed position to fully open and back to closed. During opening a valve will have a lower flow at a particular pressure than it will during closing. Increasing friction, such as by increasing sealing area between a piston and the cylinder in which it moves, results in a force which adds to the spring force in resisting opening of the valve, but also substracts from or opposes spring force in reclosing, resulting in hysteresis. The effect is most dramatic in the difference between set pressure at which the valve begins to open, and reseat pressure when the valve fully closes. A reseat pressure which is 75% or less of set pressure is not uncommon in the industry. Sealing is, of course, necessary to prevent leakage, so that some hysteresis is inevitable and, if not excessive, desirable from the standpoint of stability.
To illustrate the problem that excessive hysteresis presents, consider a system to protect a load holding circuit of 2500 psi, i.e. the pressure must not fall below this amount. The valve reseat pressure must therefore be set at 2500 psi which, because of hysteresis, results in a 3333 psi opening pressure (using a valve having a reseat of 75% of set pressure). Thus the circuit must be exposed to potentially damaging pressure peaks of up to 3333 psi to maintain the necessary load holding pressure.
In most commonly used hydraulic valves it is customary to have the inlet or high pressure at the center or end port of the valve and the lower or exhaust pressure at one or more side ports. The reverse is true with currently available differential piston relief valves, increasing the possibility of improper valve installation; an additional problem with currently available valves.